Marketplace considerations have placed significant demands on packaging for increased circuit densities. New packaging concepts have evolved to meet these requirements utilizing processes that approach the levels of device technology.
Several new packaging methods have been developed to address this need, and one of them is the use of decals.
Decal technology initially relied on utilization of an adhesive that served a dual purpose:
1) bonding of the metallurgy foil and finished images to the carrier throughout the entire process, and
2) complete release of polymer and metallurgy at time of transfer to the substrate.
Early decals were produced from a three-part laminate. The process laminate was composed of a metallurgy layer in the form of a thin metal or alloy foil which was bonded to a polymer carrier with an adhesive. The adhesive also served as a release agent permitting separation from the carrier at time of transfer. Reliability of conductor release was assured because the surface energy of the polymer layer was much less than that of either the conductor or the substrate onto which the decal was transferred. See, for example, U.S. Pat. No. 4,879,156. These early solid conductors were generated from a photolithography and etching process.
Early work rapidly revealed a limitation of such decal systems to reliably achieve adequate feature locations. Movement of the images when compared to glass artwork was noticeable, and was found to emanate from absorption and desorption of process fluids. The film carriers in some cases either expanded or contracted depending upon their placement on the absorption isotherm when they were subjected to process ambients.
Having identified the limitations of organic polymers as carriers, work centered upon identification of material that maintained dimensional integrity throughout the process and could be used as a carrier.
The material selected to replace the unstable polymer was a metal foil, which was not subject to absorption of process liquids or deformation when exposed to elevated process temperatures.
Using such a metal foil in the laminate structure, it was rapidly learned that release of metallurgy from the metal foil carrier could not be accomplished uniformly. This was a result of equal bonding of the release adhesive to both metal surfaces, i.e., the metallurgy layer and the metal foil carrier.
To overcome this characteristic, the surface energy of the metal carrier was reduced to a level much less than that of the decal metallurgy and the substrate accepting the decal. Reliable release of the conductor metallurgy from the metal carrier would then be provided. The desired bonding characteristics were achieved by coating the surface of the carrier foil with a material, such as a polyimide, to restore the release properties of the original system. The metal carrier with a two-layer release agent was found to perform as well as the original carrier with respect to conductor transfer providing improved capability for feature locational accuracy.
Utilization of additive processes provides an alternate method for formation of conductors directly on a metal carrier. Use of plating or lift-off processes in conjunction with photolithographic processes allows for conductor generation in an additive manner. This technique provides a means for attaining increased package densities due to the inherent superior image formation capability of additive processes.
A simplified decal structure was developed enabling direct release of conductors from a metal carrier without the use of release agents. This technique was applicable to conductor generation by either additive or subtractive processes, and allowed for wider range of metals and alloys to be utilized as conductors. This has been discussed in U.S. Pat. No. 4,879,156.
Another packaging method is the intaglio printing process. Images are depressed below the surface of the printing plate such that an impression from the design yields an image in relief, as disclosed in U.S. Pat. No. 4,879,156. This technique can be utilized in packaging processes by etching of the conductor pattern into the surface of the metal carrier to a depth equal to the required thickness of the finished metallurgy followed by plating of the required metallurgy to form the conductors. This technique enables generations of conductors formed in a shape defined by the image recessed into the carrier.
There are several other techniques that have been used for packaging interconnection, such as one disclosed in U.S. Pat. No. 3,541,222, where a connector screen for interconnecting adjacent surfaces of boards or modules is disclosed. The connector screen comprises of conducting connector elements that are separated by a web of nonconducting material.
A connector assembly for a circuit board testing machine is disclosed in U.S. Pat. No. 4,707,657. An electrically insulating material having circuit tracks of an electrically conductive material is arranged on opposite side surfaces. The test points are electrically insulated from each other.
A process to form Multilayered Ceramic (MLC) Substrates, having solid metal conductors, is taught in U.S. Pat. No. 4,753,694. The MLC substrate involves, forming a pattern of solid, nonporous conductors to a backing sheet having a release layer, then transferring the pattern to a ceramic green sheet.
U.S. Pat. No. 4,926,549, discloses a method of producing electrical connection members. A carrier is formed on a first electrically conductive member, holes are etched in portions of the carrier to expose the first electrically conductive member and to form recesses therein. The recesses have a diameter larger than the diameter of the corresponding hole. The respective holes formed in the carrier are filled with a second electrically conductive material, and subsequently, the first electrically conductive member is removed from the carrier, thereby, leaving a carrier having a plurality of an electrically conductive material protruding out of the upper and lower surfaces of the carrier. The carrier having the plurality of electrical conducting protrusion can then be used to connect a semiconductor device to a circuit board.
IBM Technical Disclosure Bulletin, Vol. 27, No. 3, pp. 1404-1405 (Aug. 1984) discloses a process for transferring thin-film conductor patterns to a multilayer ceramic substrate. Conductive patterns are formed on a carrier. The conductive patterns are then completely blanketed by an insulator and holes are made in the insulator to expose the upper surface of the conductive pattern. The holes are then filled with an electrically conductive material and after securely attaching this assembly to a multilayer substrate, the carrier is removed.
One of the problems that has arisen in the earlier work is the formation of gaps at the interface between the vias, such as copper vias, and the insulator sidewalls, such as ceramic sidewalls. This kind of gap allows the infiltration and entrapment of fluids, especially during the post-sinter processing. As a remedy for this problem, polyimide backfilling of the gaps has being practiced. This process has its own inherent drawbacks, such as the lack of a good bond between the polyimide and the copper vias, and the difficulty in fully curing the polyimide which has infiltrated the interior of the substrate. These inherent drawbacks cause defects in the thin film redistribution structures which are subsequently deposited on the top surface of the substrate.
The top surface metallization feature sizes are limited by the present processing techniques. Additionally, via gaps are being generated which is leading to a permeation problem in subsequent processing.
This invention provides a TFR (Thin Film Redistribution) decal structure having studs for via registration. This structure is laminated to the MLC substrate, then co-fired, thus providing "hermeticity." The process of this invention does not generate cracks in the substrates, such as in previous top surface processes. Fine line metallization is also achieved. Additionally, ready alignment of the top surface features to vias is achieved.
In this invention a process is also disclosed which does away with the thin film processing on ceramic substrates and utilizes novel etching techniques and decal structures in order to build the equivalent of thin film redistribution (TFR) prior to sintering and which would also survive the sintering cycle.
The decal structure consists of redistribution lines, C4 (Controlled Collapse Chip Connection) pads on top of solid metal studs (acting as electrical interconnects) and EC (Engineering Change) pads. In this process, the only post-sinter processing needed would be to plate Ni and Au with some sort of ball limiting structure for the C4's.
This invention also focuses upon the effort to establish solid conductors that are transferable to substrates as a viable packaging approach.
This invention also describes several unique processing methods associated with the fabrication of solid transferable electrical conductors.